A track-laying vehicle adapted to travel in a horizontal direction on the ground normally has on each side a drive assembly having in turn a horizontally elongated frame with two opposite ends and upper and lower edges extending parallel to the direction between the ends. A drive wheel is mounted at one of the ends, an idler wheel is mounted on a movable support at the other end, and a plurality of ground wheels are provided along the lower edge between the drive and idler wheels. An endless track looped around the wheels extends parallel to the direction along the idler wheels. A drive connected to the drive wheel displaces the track around the wheels. A spring unit is braced between the movable support and a support fixed on the frame and is set to allow the idler wheel to move perfectly parallel to the travel direction, which in turn is parallel to a centerline of the frame and of the track, so as to keep the track tight. Such a system is described in EP 0,052,310, DE 90 11 884, and WO 95/02,533.
With such a system as the track is pushed in, as for instance when one of the running wheels is deflected upward as the vehicle passes over a rock, the front idler wheel moves inward. In drives where the upper and lower stretches of the tracks are relatively close together, there is little room between these stretches, and much of that is occupied by the running wheels. Thus the axis of the spring unit is normally offset but parallel to the plane along which the idler wheel axis travels as it moves in and out.
The problem with this system is that it subjects the spring unit and in particular its guide structure to extreme wear because the entire assembly is twisted considerably as it is pushed in. Often the head at the end of the compression spring of the spring unit breaks off as a result of the angular force it is subjected to, requiring that expensive and difficult repairs be made.